As I wrote three months ago:

Conventional wisdom is that fringe candidates get repudiated, à la 1964 and 1972. The story isn’t so simple.

While Hillary Clinton is the consensus of most Democrats, from activists on up to the establishment, Donald Trump is the Republican candidate whom many Republicans want to avoid. . . . From this perspective, Trump’s position resembles that of Barry Goldwater in 1964 and George McGovern in 1972, two ideologically extreme candidates—Goldwater on the right and McGovern on the left—who were handicapped by strong opposition within their parties, limped through their campaigns, and got destroyed by over 20 percentage points in the general election. To add to the analogy, these candidates’ opponents—Presidents Lyndon Johnson and Richard Nixon—were, like Hillary Clinton, viewed by many voters as cynical, calculating politicians rather than inspiring leaders. Those two years, 1964 and 1972, still stand as cautionary lessons about the fate of any fringe candidate who manages to grab the presidential nomination without having secured the backing of his party’s establishment.

But Donald Trump seems to be defying political gravity. . . . What is happening? . . .

The biggest difference between 2016 and 1964/1972 has nothing to do with the candidates or the conventions or ideology or endorsements or the fracturing of political parties. It turns out that, according to many years of research from political scientists, the most important determinants of presidential elections in the past half-century or more have not been the character or political ideology of candidates, or even the strengths of their parties, but rather the state of the economy.

To emphasize the key role of the economy in setting the stage for presidential elections is not to be an economic determinist. Regression models predicting the election outcome from the economy have large error terms. But the economics-based forecast is a good starting point. . . .

And here’s what was special about 1964 and 1972: These were two of the three strongest years for the economy in the postwar era, with per-capita income growth in the 4 percent range, and the candidates running for re-election—Johnson and Nixon—won in landslides, as would be predicted. (The other strong election year in terms of economic growth was 1984, when Ronald Reagan reaped the electoral benefit.)

But 2016 is not like 1964 or 1972. The economy is slowly recovering, no longer in recession, but it is certainly not booming as in those earlier years. According to the U.S. Bureau of Economic Analysis, per-capita personal income has been growing at an annual rate of 2.5 percent during the past year and 1.2 percent averaged over the past four years. These numbers are OK but not stunning and do not foretell an electoral landslide, in either direction. Going by economic indicators, we’re looking at a close election, perhaps slightly favoring the incumbent party’s candidate, depending on how strongly one weights the most recent economic performance.

For example, here is Doug Hibbs’s forecast. I’m not saying I swear by Hibbs’s exact calculation—when I tried to set up his model, I got a forecast of 52% of the 2-party vote for Clinton, which is not quite what Hibbs reports, and one could jiggle this further by adjusting for presidential popularity (a slight plus for the Democrats), incumbent not running for reelection (a slight plus for the Republicans), and party balancing (a slight plus for the Democrats), but I buy Hibbs’s general point, not that the Democrats are guaranteed to win but that the fundamentals predict a close election with a slight edge to the Democrats and enough uncertainty to make the campaign interesting. So, yes the campaign matters but given what we know about elections it’s no surprise where the polls currently stand.